Primal meat cuts, or primals, are large cuts of meat, smaller, for example, than a side of beef, but larger than the ultimate cut that is sold at retail to the consumer. Primal cuts are prepared at the slaughter house and are then shipped to a retail meat store or an institution such as a restaurant where they are butchered into smaller cuts of meat called sub-primal meats cuts or sub-primals. Sub-primals may also be prepared at the slaughter house. When primals and sub-primals are prepared at the slaughter house, they are usually packaged in such a way that air (i.e., oxygen) is prevented from contacting the meat during shipping and handling in order to minimize spoilage and discoloration. One desirable way to package primals and sub-primals so as to protect them from degradation due to moisture loss and contact with air is to shrink package them with a packaging material that has good barrier properties. One such shrink packaging material that has good oxygen and moisture barrier properties is vinylidene chloride-vinyl chloride copolymer film.
One approach to the provision of a film for use in shrink packaging primal and sub-primal meat cuts and processed meats is to employ a multilayer film having oxygen and moisture barrier properties, one layer of which is a vinylidene chloride-vinyl chloride copolymer film. The other layer or layers of such a multilayer film are selected so as to provide the requisite low temperature properties and abrasion resistance which are lacking in vinylidene chloride-vinyl chloride film. In providing such a film, however, it must be recognized that good barrier properties, abrasion resistance, and low temperature properties are not the only requirements for a film that is to be used for shrink packaging primal and sub-primal meat cuts. The film must have been biaxially stretched in order to produce shrinkage characteristics sufficient to enable the film to heat shrink within a specified range of percentages, e.g., from about 15 to 60 percent at about 90.degree. C., in both the machine and the transverse directions.
The film must also be heat sealable in order to be able to fabricate bags from the film and in order to heat seal the open mouths of the fabricated bags when the meat cut has been placed within the bag. Additionally, the heat sealed seams of the bags must not pull apart during the heat shrinking operation, the film must resist puncturing by sharp edges such as bone edges during the heat shrinking operation, and there must be adequate adhesion between the several layers of the film so that delamination does not occur, either during the heat shrinking operation or during exposure of the film to the relatively high temperatures that may be reached during shipping and storage of the film in the summertime.
It has been proposed to prepare multilayer films, one layer of which is a vinylidene chloride-vinyl chloride copolymer and at least one other layer of which is an ethylene-vinyl acetate copolymer. For example, such films are proposed in McFedries, Jr., et al U.S. Pat. No. 3,600,267; Peterson U.S. Pat. No. 3,524,795; Titchenal et al. U.S. Pat. No. 3,625,348, Schirmer U.S. Pat. Nos. 3,567,539 and 3,607,505; and Widiger et al U.S. Pat. No. 4,247,584.
In addition, multilayer films comprising a core layer of a vinylidene chloride copolymer, wherein the latter is a copolymer of a vinylidene chloride monomer and a vinyl chloride monomer, are known, for example as disclosed in Brax et al, U.S. Pat. Nos. 3,741,253 and 4,278,738; Baird et al, U.S. Pat. No. 4,112,181; and Lustig et al Canadian Pat. No. 982,983.
Also in the prior art, cross-linking by irradiation has been used to enhance the properties of films employed in packaging operations. For example, U.S. Pat. No. 3,741,253 to Brax et al teaches a multi-ply laminate having a first ply of ethylene-vinyl acetate which is cross-linked by irradiation. The second ply and the third ply of the laminate are not irradiated. The thus-prepared laminate may then be biaxially stretched. Baird et al U.S. Pat. Nos. 3,821,182 and 4,112,181 teach a three-layer film combination which has been irradiated before stretching. Further, Bernstein et al U.S. Pat. Nos. 4,391,862 and 4,352,844 disclose co-extruding first and second polymeric layers, irradiating the co-extruded layers, joining a third layer to the second polymeric layer, and then stretching the multilayer film. Still further, Bieler et al U.S. Pat. No. 4,318,763 teaches that the seals of the bags made of multilayer film may be strengthened by cross-linking the seal area of the bag by irradiation. Finally, the entire multilayer film is preferably irradiated after biaxial orientation, as for example described in European Patent Application Publication 86-0202328.
However, it has been found that an irradiated multilayer film containing a vinylidene chloride-vinyl chloride copolymer layer discolors significantly during storage due to degradation of this layer. It is believed that discoloration of the vinylidene chloride-vinyl chloride copolymer layer is due to radiation-induced cleavage of hydrogen and chloride radicals resulting in the production of double bonds and the associated chromophores. Discoloration also occurs when this copolymer layer is exposed to elevated (above ambient) temperatures for sustained periods.
One solution to this problem is to use vinylidene chloride-methyl acrylate copolymers as the barrier layer is shrinkable multilayer films, based on the discovery that this copolymer does not significantly discolor from irradiation on sustained exposure to high temperatures. This discovery is described in European Patent Application Publication 86-0204918.
Notwithstanding this important advantage of no significant discoloration, there are processing difficulties associated with vinylidene chloride-methyl acrylate copolymer films as compared to the vinylidene chloride-vinyl chloride type. The basic problem is that this methyl acrylate copolymer is very temperature and shear sensitive during extrusion into a film. This extrusion can only be performed over a narrow temperature range without causing premature degradation of the polymer in the extruder or die. This premature degradation causes particles or gels of degraded material to exit from the extruder. These particles cause imperfections in the film and may result in a bubble break and waste of film product. During coextrusion of a multilayer film, the optimum melt temperatures of the various layers may be significantly different. Accordingly, there is a need for a wider range of methyl acrylate copolymer extrusion process temperatures to enable extrusion with other copolymers. Even at optimum extrusion temperatures, a certain amount of degraded material will form in the extruder and die. It is important to retain this material in the extrusion system in the form of a thin layer on the extruder and die walls as long as possible between equipment clean outs.
An object of this invention is to provide an improved barrier layer-containing multilayer shrinkable film which does not discolor on irradiation or sustained exposure to elevated temperatures, but which avoids processing difficulties due to temperature/shear sensitivity during double bubble to coextrusion.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.